1. Field of the Invention
The present invention relates to an apparatus and method of observing a crystalline specimen by using a diffraction image observed by an electron microscope.
2. Description of the Related Prior Arts
An example of a conventional technique of measuring a strain from a change amount relative to reference positions of a distance between spot patterns obtained by Fourier transforming an arbitrary micro area in a lattice image observed by enlarging a crystalline specimen by about 200,000 times with a transmission electron microscope, and mapping the strains is disclosed in Japanese Unexamined Patent Application No. 2000-65762 (cited document 1) under the title of xe2x80x9ccrystal strain measuring method, crystal strain measuring apparatus, and recording mediumxe2x80x9d.
A technique of automatically extracting intersecting positions of HOLZ (High Order Laue Zone) lines appearing in a CBED (Convergent Beam Electron Diffraction) image formed by irradiating a specimen with a condensed electron beam by image processing and displaying the position change amount as a strain amount strength in colors or by using contour lines is disclosed in Japanese Unexamined Patent Application No. 10-162728 (cited document 2) under the title of xe2x80x9cMethod and apparatus for evaluating lattice strain by using condensed electron beam diffraction figurexe2x80x9d. An HOLZ line is a kind of a high order diffraction image and, it is conventionally known that its line spacing corresponds to a lattice plane spacing of a crystal in a specimen. That is, a stress and a strain of the crystal in a specimen can be obtained by conversion from a change in the HOLZ line spacing. Since the condensed electron beam emitted to a specimen is usually condensed to a beam diameter of 10 nmxcfx86 or less, the strain and stress can be measured with a resolution of the order of nm.
The measuring method disclosed in Cited Document 1 has a problem such that sensitivity to stress and strain is insufficient. Specifically, lattice widths in a lattice image are larger than a half of lattice spacing, so that if a change in the lattice position is not about 5% or more, a strain amount is not reflected in a Fourier transform image. The strain amount in a silicon semiconductor device is about 3% at the maximum (in many cases 1% or less), which is lower than the detection limit of the measuring method. Consequently, the measuring method has a subject to increase the sensitivity so that a smaller strain or stress amount can be measured.
The measuring method by using a convergent electron beam diffraction disclosed in Cited Document 2 has the following problem. In a high-order diffraction pattern used by the CBED method, a very small lattice strain is reflected in a diffraction pattern very sensitively, so that stress and strain sensitivity is very high. An electron beam for measurement is conversed to 10 nm xcfx86 or less, and spatial resolution is very high. However, since a high-order diffraction pattern of a low intensity appearing in diffraction spots of a transmitted low-order diffraction pattern has to be observed, it is necessary to make a specimen very thin and reduce an inelastic scattering background by using an energy filter. In such a manner, the sample thickness is suppressed to about 100 nm or less. Since the sample is processed to be very thin, the stress and strain are largely relaxed in the film thinning process, and it caused a problem such that it is very difficult to obtain information of the stress and strain originally held in the specimen. When the energy filter is necessary, a problem of higher cost occurs. Further, since high-order diffraction information is used, there is another problem such that the method is highly susceptible to an influence of damage caused by irradiation of an electron beam. Therefore, the measuring method is requested to be developed to a measuring method of observing a low-order diffraction image in a thicker sample.
In short, conditions required to evaluate a stress and a strain in a semiconductor device are realization of the method of observing a low-order diffraction image of a thicker specimen with high spatial resolution and high sensitivity to stress and strain. If even one of the conditions is not satisfied, the technique cannot be practical.
An object of the invention is therefore to solve the problems of the conventional techniques and to embody a practical stress and strain measuring technique to which a method of observing a low-order diffraction image of a thicker observation specimen with high spatial resolution and high sensitivity to stress and strain can be applied.
More specifically, an object of the invention is to provide a sample observation method using an electron beam, capable of embodying the practical stress and strain measuring technique, and a sample observation apparatus suitable for carrying out the method.
Another object of the invention is to provide a technique capable of visually two-dimensionally displaying a stress/strain distribution in a specimen at high resolution on the basis of a measurement result obtained by using the specimen observation method and apparatus.
Further another object of the invention is to establish peripheral techniques such as an inspection algorithm for a sampling inspection in a manufacturing line of a semiconductor device suitable for use in development of a stress reducing process for manufacturing the semiconductor device on the basis of a measurement result obtained by using the sample observation method and apparatus, the configuration of an inspection system capable of establishing the inspection algorithm, and sampling of an inspection sample.
To achieve the objects of the invention, the invention provides an observation method and apparatus using an electron beam, which has characteristic configuration as described below.
First, to realize observation of a fine crystal structure in a specimen at high resolution, according to the invention, a specimen is irradiated with an electron beam and a diffraction image is obtained from the electron beam diffracted in the specimen. Particularly, to irradiate a specimen with a convergent electron beam having a diameter of 10 nm xcfx86 or less, an electron beam optical path according to a so-called nano-diffraction method for converging an electron beam to a very small beam diameter by a condenser aperture of an electron microscope is employed. In the case of employing the nano-diffraction optical path, a sample can be irradiated with a narrow parallel electron beam. The parallelism of the irradiation electron beam at this time is set to 0.5 mrad or less. This is largely different from the CBED method which emits a not-parallel electron beam at a large irradiation angle of 10 mrad or larger. By irradiating a specimen with a parallel electron beam as described above, the spot diameter of an obtained diffraction image becomes sufficiently small. The lattice plane spacing of a crystal in a specimen can be measured from a spot spacing with high precision, and measurement sensitivity to stress and strain in the sample crystal can be improved. To observe a thick specimen, a diffraction spot (low-order diffraction spot) formed by an electron beam diffracted at the 222 orientation or lower, at which an electron beam absorption is little and a diffraction electron intensity is strong in a specimen is observed. It enables a thicker specimen to be observed, and relaxation of a stress and a strain in a sample which occurs at the time of thinning a specimen can be largely suppressed. By forming a low-order diffraction image by using a nano-diffraction optical path, a stress and a strain in a specimen can be measured at high resolution of 10 nm or less and with high sensitivity of 0.5% or less. Moreover, a sample which is ten times or more as thick as that can be observed by a conventional observation method can be measured. Realization of measurement of a thicker specimen at high resolution and high sensitivity is a very big advantage of the invention.
In the invention, to visually display the stress and strain distributions in a semiconductor device sample obtained by the high-resolution high-sensitivity measuring method, the following technical means is taken. The above-described diffraction image measurement is executed, for example, with respect to tens of spots in a 1-bit transistor device. A similar diffraction image measurement is executed also on a substrate portion of the same bit. A lattice plane spacing as a reference is measured in the substrate portion, and the difference between the reference lattice plane spacing and a lattice plane spacing measured at each measurement spot is obtained and used as a strain amount at the measurement spot. A stress in a specimen is in a proportional relationship with the strain amount of the lattice plane spacing, and its proportionality constant is a value peculiar to the elements in the specimen and a direction of crystal plane and can be preliminarily calculated. Therefore, by two-dimensionally superimposing a change between the diffraction spot in the direction perpendicular to the substrate and the diffraction spot in the direction horizontal to the substrate onto an electron microscope photograph, a two-dimensional distribution of stress and strain can be visually displayed in correspondence with an image of the structure in the device.
To establish a stress reducing process, a transistor device of a bit to be observed is extracted from a wafer on a semiconductor fabrication line and measurement similar to the above is performed. By extracting a specimen before and after a process to which attention is paid, a state of accumulation of a stress and a strain in the portion can be grasped. After performing a process A, a process B is executed on a wafer, and a process C is executed on another wafer, thereby fabricating devices. A device as a specimen is extracted from the wafer subjected to the process B, and a device as a specimen is extracted from the wafer subjected to the process C. The stress and strain amounts of each of the samples (devices) are measured, thereby enabling advantages and disadvantages from the viewpoint of stress and strain of the processes B and C to be grasped. By repeating it, a preferable stress reducing process can be found out.
Other objects, configurations, and action and effects produced by the configurations will become apparent from the following detailed description of embodiments.